1. Field of the Invention
The present invention relates to a tape adhering method, and to a tape adhering apparatus for carrying out the tape adhering method, which are used to adhere a tape such as a dicing tape on the back surface of a wafer or a surface protection tape on the front surface of the wafer.
2. Description of the Related Art
In the field of semiconductor production, the trend is toward an ever-increasing size of the wafer. On the other hand, the wafer is becoming thinner and thinner to increase the density. In order to reduce the thickness of the wafer, the back surface of the semiconductor wafer is ground. In the process, a surface protection tape is adhered on the front surface of the wafer to protect the semiconductor elements formed on the front surface of the wafer.
The wafer formed with semiconductor elements on the surface thereof is finally cut into cubes by a dicing machine. During the dicing process, the wafer is integrated with a frame by the dicing tape adhered to the back surface of the wafer. Then, the wafer is “half cut” to the intermediate part of the wafer by the dicing blade of the dicing machine, or the wafer is “full cut” completely while cutting to the intermediate part of the dicing tape. Before adhering the dicing tape on the back surface of the wafer, the die attach film tape (DAF tape) may be adhered on the back surface of the wafer. The DAF tape functions as an adhesive on the die bottom at the time of die bonding after dicing. Alternatively, a dicing tape having a die attach film (DAF) having substantially the same shape as the wafer may be used. Unless otherwise specified, the dicing tape described below also contains the die attach film.
Generally, the tape adhering apparatus used for adhering the surface protection tape or the dicing tape comprises an adsorption table for adsorbing the wafer and a roller adapted to move along the upper surface of the wafer adsorbed to the adsorption table. The tape to be adhered is supplied between the roller and the wafer. The roller is adapted to move in parallel along the upper surface of the wafer on the one hand and in the direction perpendicular to the upper surface of the wafer on the other hand. The adsorption table, on the other hand, is immobile and adsorbs the wafer.
FIG. 10a is a diagram showing the relation between the force exerted between the roller and the adsorption table (ordinate) and the distance of the roller from one end of the wafer (abscissa) in an ordinary tape adhering apparatus. FIG. 10b is a diagram showing the relation between the pressure applied to the wafer (ordinate) and the distance of the roller from one end of the wafer (abscissa) in FIG. 10a. In these drawings, the distance from one end of the wafer on the straight line passed through the center of the wafer is designated as x, and the radius of the wafer as r. As shown in FIG. 10a, the force F(x) exerted between the roller and the adsorption table of an ordinary tape adhering apparatus is set substantially constant over the whole adhering process.
The wafer on which the surface protection tape or the dicing tape is adhered, i.e. the silicon wafer used for fabricating a semiconductor is normally circular in shape. The roller starts to contact the wafer at one end of the wafer and moves toward the other end. Normally, the pressure P(x) exerted on the wafer is determined based on the roller located at the center of the wafer. In the case where the roller is located at about the center of the wafer (x=r), as shown in FIG. 10b, therefore, the desired pressure P1 is obtained. As the roller moves toward one end of the wafer, however, the pressure gradually increases and becomes almost infinitely large at the ends of the wafer (x=0 and x=2r). This is considered to be due to the fact that the wafer is circular in shape and therefore the area of the wafer in contact with the roller changes when the roller slides, i.e. the contact area is smaller at the ends and larger at the center of the wafer and the fact that the force exerted on the wafer is constant as shown in FIG. 10a. 
An explanation is given for a case in which the pressure imparted when the surface protection tape or the dicing tape is adhered on the wafer changes as shown in FIG. 10b. FIG. 11a is a sectional view, taken along the direction in which the roller proceeds, with the surface protection tape 500 adhered on the surface 21 of the wafer 20 formed with a plurality of semiconductor devices 25 by an ordinary tape adhering apparatus. In similar fashion, FIG. 11b is a sectional view, taken along the direction in which the roller proceeds, with the dicing tape 51 adhered on the back surface 22 of the wafer 20 by an ordinary tape adhering apparatus. The dicing tape 51 contains the DAF 52. In these drawings, the roller (not shown) proceeds from a first end 28 to a second end 29 of the wafer 20. The surface protection tape 500 and the dicing tape 51 are comparatively soft. In the case where the pressure for adhering any of these tapes undergoes a change as shown in FIG. 10b, therefore, the portion of the tape under a high pressure is flattened. Specifically, as shown in FIGS. 11a and 11b, the surface protection tape 500 and the dicing tape 51 are thick at the center of the wafer 20 and tend to decrease in thickness at the ends 28, 29 of the wafer 20 along the direction of roller movement. In the case where the surface protection tape 500 or the dicing tape 51 is adhered on the wafer 20, therefore, the inconveniences described below occur.
First, with reference to FIG. 11a, the inconveniences caused when the surface protection tape 500 is adhered on the surface 21 of the wafer 20 are explained. In this case, the surface protection tape 500 at the ends 28, 29 of the wafer 20 is considerably compressed as compared with the center of the wafer 20 and the neighborhood thereof. At the ends 28, 29 of the wafer 20, therefore, the adhesive contained in the surface protection tape 500 is squeezed out from the wafer or intrudes into the details of the semiconductor elements 25. In detaching the surface protection tape 500 at the end of back grinding, therefore, the residue of the surface protection tape 500 may remain around the semiconductor element 25 in the neighborhood of the ends 28, 29. Also, when grinding (back grinding) the back surface 22 of the wafer 20 after adhering the surface protection tape 500, the surface protection tape 500 is directed downward. In the process, the thickness of the whole wafer 20 including the surface protection tape 500 at about the center of the wafer 20 is larger than the thickness at about the ends of the wafer 20. In the case where the wafer is back-ground under this condition, the thickness of the wafer 20 at about the ends 28, 29 is increased beyond that of the central portion thereof. This thickness difference remains as it is even after dicing, resulting in size variations in the formed semiconductor devices.
In FIG. 11b, the dicing tape 51 containing the DAF 52 is adhered to the back surface 22 of the wafer 20, after which a plurality of grooves 40 are formed on the surface 21 of the wafer 20 by a dicing device (not shown). In this case, the grooves 40 are formed by operating the dicing device with the intention of performing a full-cut operation. A part (chip) of the wafer including the semiconductor elements 25 in the neighborhood of the ends 28, 29 of the wafer 20 may still be half cut. Further, the thickness of the DAF 52 on the dicing tape 51 functioning as an adhesive at the time of die bonding varies between the central portion and the ends 28, 29. As a result, the chip thickness is varied after the die bonding or the adhesion of the DAF 52 may change. Also, the DAF portion may not be cut off.
In addition, though not shown, the chip may inconveniently assume different thicknesses at the central portion and the ends 28, 29 of the wafer 20 as in the case where back-grinding is conducted after dicing.
In order to obviate the inconveniences when adhering the dicing tape to the back surface of the wafer as described above, a substantially uniform pressure is required to be imparted over the whole wafer 20. Japanese Unexamined Patent Publication No. 2002-134438discloses a tape adhering method in which the dicing tape located between the roller and the wafer is adhered on the wafer by moving the roller along the upper surface of the wafer supported on a fixed table. In Japanese Unexamined Patent Publication No. 2002-134438, the force exerted on the wafer by moving the roller toward the fixed table can be adjusted by adjusting the roller position. As a result, an equal pressure can be imparted over the entire wafer. Also, the dicing tape 51 which may include the DAF 52 is adhered with the same thickness at about the central portion and the ends 28, 29 of the wafer 20 Japanese Unexamined Patent Publication No.2002-134438.
As long as the force is applied by the roller to the wafer, the tape located between the roller and the wafer is pulled in a slanting direction to the direction in which the roller proceeds. In this case, the tension applied to the tape is divided into a component in the direction in which the roller proceeds and a component in the direction opposite to the force applied to the wafer by the roller. Specifically, in Japanese Unexamined Patent Publication No. 2002-134438, the force of the roller acts in the direction toward the wafer, while the tension applied on the tape contains the force in the opposite direction. In the case where the force is applied to the wafer by moving the roller toward the table, therefore, the tension of the tape located between the roller and the wafer is also required to be taken into consideration.
The tape adhered to the wafer, however, is not necessarily the dicing tape but may be the surface protection tape. The tape characteristics such as the thickness, hardness and the bonding temperature are naturally varied from one tape to another, and therefore the tension of the tape cannot be easily determined.
Specifically, the tension of the tape contains the component opposite to the direction in which the force is applied to the wafer by the roller, and therefore, in the case where the roller is pressed against the wafer via the tape, the response of the pressure applied by the roller to the wafer become low, thereby making it difficult to perform a quick control operation. Further, in view of the fact that the weight of the roller itself is imposed on the wafer, it is actually difficult to accurately control the force applied on the wafer by the roller.
This invention has been developed in view of the situation described above, and the object thereof is to provide a tape adhering method, and a tape adhering apparatus for carrying out the method, in which an accurate, quick control operation is performed to impart an equal pressure over the whole wafer at the time of adhering the tape.